Organic fi eld-effect transistors (OFETs) are attractive building blocks for low-cost electronic devices such as radio-frequency identifi cation (RFID) tags, sensors, electronic paper, and backplane circuits for active-matrix displays. [1][2][3][4][5][6] Low-voltage operation of OFETs is necessary for practical applications, hence the need to develop gate dielectrics with a high areal capacitance. [ 7 , 8 ] To this end, ultrathin dielectric layers have been demonstrated based on self-assembled-monolayer (SAM) chemistry. [ 7 , 9-11 ] In these cases, the formation of the dielectric layer relies on specifi c surface chemistries and therefore these approaches may not be suitable for the deposition of blanket layers on arbitrary substrates. For the same reason, the robustness and yield may be problematic due to the critical roles that surface cleanliness and roughness play on binding. On the other hand, inorganic. high-κ metal oxide dielectrics, which are attractive due to their high dielectric constant, have been fabricated by various methods such as metal anodization, [ 10 , 12 ] vacuumbased deposition, [13][14][15] and sol-gel chemistry. [ 16 ] An advantage of an oxide dielectric is the ability to functionalize the surface with a variety of SAMs that are often found to be benefi cial to charge transport in OFETs, or can be used to control the threshold voltage. Although inorganic/SAM hybrid dielectrics provide high capacitance and low leakage current, so far they have either required vacuum processing or relatively hightemperature anneals ( T > 200 ° C), making them incompatible with low-cost solution processing on fl exible substrates. Here, we describe the fabrication of low-voltage polymeric OFETs where the dielectric and the semiconductor are deposited from solution at room temperature. Zirconium oxide (ZrO x ) was deposited via a sol-gel process and fully cured by UV irradiation under ambient conditions, eliminating the need for a hightemperature anneal. In order to reduce the leakage current and make the dielectric compatible with organic semiconductors, the ZrO x fi lm was functionalized with a layer of octadecylphosphonic acid (ODPA). As a result, a high-performance ( μ = 0.2 cm 2 V − 1 s − 1 ), low-voltage ( | V GS | < 3 V), and high-on-off-ratio (10 5 -10 6 ) polymer OFET processed entirely from solution at room temperature is demonstrated using poly(2,5-bis(3-tetradecylthiophen-2yl(thieno[3,2-b]thiophene) (PBTTT-C 14 ) as the semiconductor. The fi eld-effect mobility was exclusively limited by the fact that the processing occurred entirely at room temperature and therefore the semiconductor was not annealed into its most-ideal microstructure. [17][18][19] A staggered, bottom-gate device structure was fabricated ( Figure 1 a ) using ZrO x as the gate dielectric, due to its high dielectric constant ( κ = 25) and wide bandgap ( E g = 5.8 eV). [ 20 ] Typical ZrO x -deposition methods include atomic-layer deposition, [ 21 ] sputtering, [ 22 ] and e-beam evaporation. [ 23 ] These methods, however, require...
Abstract— Methods used to deposit and integrate solution‐processed materials to fabricate TFT backplanes by ink‐jet printing are discussed. Thematerials studied allow the development of an all‐additive process in which materials are deposited only where their functionality is required. The metal layer and semiconductor are printed, and the solution‐processed dielectric is spin‐coated. Silver nanoparticles are used as gate and datametals, the semiconductor used is a polythiophene derivative (PQT‐12), and the gate dielectric is an epoxy‐based photopolymer. The maximum processing temperature used is 150°C, making the process compatible with flexible substrates. The ION/IOFF ratio was found to be about 105−106, and TFT mobilities of 0.04 cm2/V‐sec were obtained. The influence of surface treatments on the size and shape of printed features is presented. It is shown that coffee‐stain effects can be controlled with ink formulation and that devices show the expected pixel response.
Current electronic portal imaging devices (EPIDs) based on active matrix flat panel imager (AMFPI) technology use a metal plate+phosphor screen combination for x-ray conversion. As a result, these devices face a severe trade-off between x-ray quantum efficiency (QE) and spatial resolution, thus, significantly limiting their imaging performance. In this work, we present a novel detector design for indirect detection-based AMFPI EPIDs that aims to circumvent this trade-off. The detectors were developed using micro-electro-mechanical system (MEMS)-based fabrication techniques and consist of a grid of up to approximately 2 mm tall, optically isolated cells of a photoresist material, SU-8. The cells are dimensionally matched to the pixels of the AMFPI array, and packed with a scintillating phosphor. In this paper, various design considerations for such detectors are examined. An empirical evaluation of three small-area (approximately 7 x 7 cm2) prototype detectors is performed in order to study the effects of two design parameters--cell height and phosphor packing density, both of which are important determinants of the imaging performance. Measurements of the x-ray sensitivity, modulation transfer function (MTF) and noise power spectrum (NPS) were performed under radiotherapy conditions (6 MV), and the detective quantum efficiency (DQE) was determined for each prototype SU-8 detector. In addition, theoretical calculations using Monte Carlo simulations were performed to determine the QE of each detector, as well as the inherent spatial resolution due to the spread of absorbed energy. The results of the present studies were compared with corresponding measurements published in an earlier study using a Lanex Fast-B phosphor screen coupled to an indirect detection array of the same design. The SU-8 detectors exhibit up to 3 times higher QE, while achieving spatial resolution comparable or superior to Lanex Fast-B. However, the DQE performance of these early prototypes is significantly lower than expected due to high levels of optical Swank noise. Consequently, the SU-8 detectors presently exhibit DQE values comparable to Lanex Fast-B at zero spatial frequency and significantly lower than Fast-B at higher frequencies. Finally, strategies for reducing Swank noise are discussed and theoretical calculations, based on the cascaded systems model, are presented in order to estimate the performance improvement that can be achieved through such noise reduction.
The operational stability of organic thin-film transistors (OTFTs) comprising bilayer polymer dielectric of poly(methylsilsesquioxane) (pMSSQ) and either the epoxy resin SU-8 or poly(4-vinyl phenol) was examined. Although not in direct contact with the semiconductor materials, the bottom dielectric layer did affect OTFT stability through water ion movement or charge injection inside the bottom dielectrics. In the comparison between our best polymer dielectric pMSSQ/SU-8 to the silicon oxide dielectric, the result emphasized that, at equal initial charge concentration, polymer dielectrics did not alleviate threshold-voltage shift but did maintain more stable current due to the lower gate capacitance than silicon oxide.
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